This invention relates to the operation of implantable pacers, and more particularly to means for varying operating parameters of an implanted pacer and for monitoring pacer operation.
Various types of pacers are known for implantation in the body in order to control the operation of certain organs through the application of timed electrical pulses. The best known of these devices is the cardiac pacer which comprises a self-contained, battery powered pulse generator for applying timed, stimulating pulses to heart tissue of the wearer. Initially, pacers of this description operated at a fixed, invariant rate. Later the so-called "demand" type of pacer was developed and has generally superseded the fixed-rate pacer. With demand type pacers, pulses are produced only when a naturally-occuring heart-beat is not detected within some maximum time period. Various operating parameters of the pacer are of particular interest: the pulse width or duration; pulse amplitude; and sensitivity of the pacer to naturally-produced pulses are all of interest. It is a well known fact that some or all of these parameters, as well as the minimum pulse rate, should be tailored to suit an individual wearer. For this reason pacers are often constructed so that they may be adjusted before implantation to allow a physician to modify certain pacer characteristics as appropriate for a given wearer.
Although pacer adjustments are generally correct, in some cases it would be advantageous to be able to modify certain pacer operating characteristics after the pacer has been in use for some time. Accordingly, pacers have been designed which could be adjusted at will. In one approach, a pacer was provided with a movable magnetic element which could be manipulated by bringing another magnet close to the wearer's body, and manipulating the magnet adjacent the pacer. Still other approaches have been devised wherein sonic, ultrasonic, or radio frequency signals from an external signal generator are received by an implanted pacer, and the signals used to modify certain pacer operating characteristics. Examples of systems of this description are contained in U.S. Pat. No. 3,631,680; Lopin; U.S. Pat. No. 3,718,909; ; Greatbach; U.S. Pat. No. 3,805,796, Terry et al; U.S. Pat. No. 3,833,055; Wingrove; and U.S. Pat. No. 3,945,387; Adams. Most systems of the types disclosed in these patents comprise counters and decoders which count the number of signal pulses which are received and convert them into binary numbers which represent a desired mode of operation.
The application of a variable number of signal pulses is, for a number of reasons, not considered to be a totally satisfactory method of adjusting the parameters of an implanted pacer. Unless bulky, sophisticated checking systems are utilized there is no effective way for the pacer to recognize whether a proper number of pulses has been received. Proper adjustment of the pacer can only be verified by monitoring its operation; in other words, a trial and error procedure. If a spurious pulse, for instance one arising due to electromagnetic noise, occurs a significant change in the data will be effected. The same is true for the dropping of a pulse during the generation, transmission or detection of the data. For example, in one coding system 16 received bits would be decoded to produce the binary number 10000. Dropping a signal bit, so that only 15 pulses or accounts are received, would then produce the binary number 01111. Still further, according to prior art systems of the type illustrated above additional complete stages or data registers would be required for each additional process variable to be controlled.
It is also highly advantageous for a physician or technician to be able to easily monitor pacer operation. Ordinarily, monitoring of a pacer is accomplished by attaching electrodes to the body of the pacer wearer. Pacer pulses are then inferred from electrical signals picked up from the body tissue of the wearer. The pacer pulses, which thus must traverse substantial amounts of body tissue, may be substantially degraded; further, the signals may be difficult to accurately detect and interpret. It is apparent, moreover, that the physical connection of leads or sensors to an implanted pacer in order to obtain a direct reading is not practical. In addition, no means are presently known for eliciting a desired response from a pacer in order to check a mode of operation which the pacer does not ordinarily exhibit.
From the foregoing it should be understood that it would be desirable to provide means for easily varying the operating parameters of an implanted pacer at will; and moreover to monitor various pacer operating characteristics without the need for cumbersome and time-consuming electrode attachment procedures.
It is therefore an object of the present invention to provide a system for modifying the operating characteristics of an implanted pacer.
Another object of the invention is to provide improved means for monitoring the operation of an implanted pacer.
Still another object is to provide means for eliciting the specific responses from an implanted pacer, and monitoring the ensuing pacer operation.